Polyurethanes which are flame-retardant are highly desired and required for many applications especially as the pressure for flame-retarded products increases in the legislative area. In general, imparting a specified level of flame-retardancy to polyurethane polymers can be accomplished with relative ease by compounding with flame-retardant compounds after the polycondensation reaction leading to the formation of the polymer. Flame-retardants, classified by function, fall logically into three major classes: reactive, additive and synergistic.
The most widely used flame-retardants for polyurethanes are the additive type such as the halogenated aliphatic hydrocarbons in conjunction with synergists such as antimony compounds; see, for example, Modern Plastics Encyclopedia, p. 228, 1974-1975. The synergist enhances the effectiveness of the flame-retardant and thereby provides higher levels of flame-retardance at moderate flame-retardant use levels without significantly compromising other desirable physical properties; see Lyons, J.W., "The Chemistry & Uses of Fire-Retardants," p. 364, 1970; John Wiley & Sons, Inc.
Flexible polyurethanes, both solid and foamed, are more difficult to flame-retard while maintaining an acceptable balance of properties. The flexible polyurethane molecules are longer and more delicate than the rigid molecules and are therefore more sensitive to the loss of key physical properties with minor changes in composition. The flexible molecules are also more flammable than the molecules of rigid polyurethanes because they contain a higher percentage of flammable components such as aliphatic polyethers, 70% versus 50% or less for the rigid polyurethanes. An additional drawback of flame-retardant flexible polyurethanes is the phenomenon of dripping of the melted polyurethane polymer while it is burning during and shortly after ignition. The flaming droplets may spread the flame beyond the site of initial ignition. Indeed, in some compositions, the formulation would be self-extinguishing except for the fact that the dripping away of the melted polymer constantly exposes a fresh new surface of polymer to the fire, and the fire continues to progress.